The calcium-sensing receptor (CaSR) is a G-protein-coupled receptor (GPCR) that signals through the activation of phospholipase C, increasing levels of inositol 1,4,5-triphosphate and cytosolic calcium. The CaSR belongs to the subfamily C of the GPCR superfamily, which also includes receptors for glutamate, gamma aminobutyric acid (GABA), pheromones and odorants that all possess a very large extracellular domain. This domain is highly negatively charged and is involved in binding of calcium and other positively charged molecules. The CaSR is found in the parathyroid glands but has also been identified in the brain, intestine, pituitary, thyroid glands, bone tissue and kidneys. In the parathyroid glands, the CaSR is activated by small increases in extracellular ionized calcium, which inhibits parathyroid hormone (PTH) release from within the stored intracellular granules [Brown, E. M. Calcium-Sensing Receptor. Primer of the Metabolic Bone Diseases and Disorders of Mineral Metabolism Fifth Edition, 2003 by American Society for Bone and Mineral Research, Chapter 17, p. 111.; Drueke, T. E. Nephrol Dial Transplant (2004) 19, v20-v26].
In addition to endogenous ligands, small molecule allosteric activators of the CaSR (“calcimimetics”) have been developed [Urena, P.; Frazao, J. M. Calcimimetic agents: Review and perspectives. Kidney International (2003), 63, pp. s91-s96; Soudijn, W. et al. Allosteric modulation of G protein-coupled receptors: perspectives and recent developments. DDT (2004), 9, 752-758].
The binding site of known calcimimetics is believed to be located in the seven-transmembrane domain of the receptor [Petrel, C. et al. Journal of Biological Chemistry (2004), 279, 18990-18997].
Calcimimetics have already been shown to be commercially useful for the treatment of hyperparathyroidism (HPT): The calcimimetic compound Cinacalcet® [Balfour, J. A. B. et al. Drugs (2005) 65(2), 271-281; Linberg et. al. J. Am. Soc. Nephrol (2005), 16, 800-807, Clinical Therapeutics (2005), 27(11), 1725-1751] has recently been launched for the treatment of secondary HPT in chronic kidney disease patients on dialysis and for the treatment of primary HPT in patients with parathyroid carcinoma.
Thus, proof of concept for activators of calcium sensing receptor (CaSR) in humans has been achieved and the clinical relevance is already well established.
In chronic kidney disease hypocalcemia results from a disturbance in renal phosphorus handling and decreased formation of 1,25(OH)-2-VitD. In response, the PTH secretion is increased resulting in a condition referred to as secondary HPT. Primary HPT is a hypercalcemic disorder that results from excessive secretion of PTH usually caused by parathyroid adenoma or primary parathyroid hyperplasia.
Other calcimimetic compounds were for example described in WO 94/018959, WO98/001417, WO05/065050, WO03/099814, WO03/099776, WO00/21910, WO01/34562, WO01/090069, WO97/41090, U.S. Pat. No. 6,001,884, WO96/12697, EP1203761, WO95/11221, WO93/04373, EP1281702, WO02/12181, WO04/069793, US2004242602, WO04/106296 and WO05/115975.
The calcimimetic activity corresponds to the ability to produce or induce biological responses observed through variations in the concentration of extracellular calcium ions (Ca2+)e and extracellular magnesium ions (Mg2+)e.
(Ca2+)e and (Mg2+)e ions play a major role in the body since they regulate calcium homeostasis on which the vital functions of the body depend. Thus, hypo- and hypercalcemia, that is to say conditions in which (Ca2+)e ions are below or above the mean threshold, have a major effect on many functions, such as cardiac, renal or intestinal functions. They deeply affect the central nervous system [Chattopadhyay et al. Endocr. Review, (1998)].
CaSRs are proteins which are sensitive to (Ca2+)e and (Mg2+)e ions, and are present in the parathyroid and thyroid glands, the kidney, the intestine, the lungs, bone cells, the brain, the spinal cord, the pituitary gland, the stomach and keratinocytes [Brown et al, Nature, (1993); Ruat et al, Proc. Natl. Acad. Sci., USA, (1995); Brown et al, Ann. Rev. Med., (1998)]. These proteins are encoded by a single gene isolated from various animal species. They belong to the family of G protein-coupled receptors with seven transmembrane domains, and exhibit structural homologies with metabotropic glutamate receptors, GABA receptors, and hypothetical pheromone and taste receptors. Activating or inhibitory mutations of the genes in humans are responsible for extremely serious genetic diseases which cause hypocalcemia or hypercalcemia [Pollack et al, Cell, (1993); Pollack et al, Nature Genetic, (1994); Brown et al, Ann. Rev. Med., (1998)]. The functions associated with the expression of these proteins in tissues are not yet all known and are the subject of a very great deal of research activity, particularly with regard to the CaSRs present in the parathyroid and thyroid glands, the kidney, the intestine, the spinal cord, the brain and bone cells.
In the parathyroid gland, the CaSRs modulate the secretion of parathyroid hormone (PTH), which is the main regulator of calcium homeostasis: an increase in (Ca2+)e ions in the serum will activate the CaSRs present on the cells of the parathyroid gland and decrease secretion of the PTH hormone.
The complementary DNA encoding rat CaSR has been isolated from a rat striatum cDNA library [Ruat et al, Proc. Natl. Acad. Sci., (1995)]. This receptor is identical, in terms of its amino acid sequence, to that expressed in the other tissues. Transfected Chinese hamster ovary (CHO) cells expressing rat CaSR(CHO(CaSR)) have been characterized and the chemical signals (second messengers) induced by activation of this receptor have been analyzed. Thus, a biochemical test for measuring the accumulation of tritiated inositol phosphates, [3H]IPs, in response to activation of the receptor has been developed [Ruat et al, J. Biol. Chem., (1996); Ferry et al, Biochem. Biophys. Res. Common., (1997)].
It has been shown that Ca2+ and Mg2+ ions, but also Ba2+ ions, within millimolar concentration ranges, stimulate CaSRs. Activation of CaSRs might be induced in the brain by β-amyloid peptides, which are involved in neurodegenerative diseases such as Alzheimer's disease [Ye et al, J. Neurosci. Res. (1997)].
Disturbance of CaSR activity is associated with biological disorders such as primary and secondary hyperparathyroidism, osteoporosis, cardiovascular, gastrointestinal, endocrine and neurodegenerative diseases, or certain cancers in which (Ca2+)e ions are abnormally high.
Secondary hyperparathyroidism is observed in chronic renal failure and is characterized by hyperplasia of the parathyroid glands and an increase in circulating PTH. The renal failure is also accompanied by renal osteodystrophy, e.g. osteitis fibrosa, osteomalacia, adynamic bone disease, or osteoporosis. The disorders are characterized by either high or low bone turnover.
Osteoporosis is a multifactor disease which depends in particular on age and sex. While menopausal women are very greatly affected, osteoporosis is increasingly proving to be a problem in elderly men, and, for the moment, no really satisfactory treatments exist. Its social cost may become even heavier in the years to come, particularly in our European society where life expectancy is becoming longer. Osteoporosis is currently treated with estrogens, calcitonin or biphosphonates which prevent bone resorption without stimulating bone growth. More recent data demonstrate that intermittent increases in PTH or in derivatives thereof are effective in the treatment of osteoporosis and make it possible to remodel bone by stimulating bone formation [Whitfield et al, Drugs & Aging, (1999), Whitfield et al, Calc. Tissue Int., (1999)]. This new therapeutic approach for treatment of osteoporosis appears to be very advantageous, although major problems are associated with the use of PTH hormone, such as the route of injection, but also the appearance of tumors, observed recently during clinical trials in humans. Intermittent secretion of endogenous PTH can be obtained by blocking the calcium sensing receptor. The blocking of PTH secretion with CaSR agonists may be followed by a rapid increase in PTH (rebound effect), which is then beneficial in the treatment of osteoporosis.